The evolution of climate change sets new design paradigms for the aeronautic industry. The imperative to reduce CO2 emissions necessitates the development of disruptive aircraft configurations. For regional flights, one exploration is to implement to the classical tube and wing aircraft, innovative powertrain solutions such as the hybrid one. While this innovation is actively studied in the research world, its uptake is still slow in the industrial one. Indeed, although hybrid electric propulsion appears as a great asset in the fleet decarbonization, the maturity of such technologies, and their modelisation in preliminary design in the literature, does not bring confidence to the industrials to develop such aircraft. To bridge this crucial gap, this work proposes a robust, high-fidelity Multidisciplinary Design Analysis and Optimization (MDAO) framework tailored for the conceptual design of hybrid-electric regional aircraft. The methodology centers on implementing a parallel hybrid power chain on a reference regional commuter (the ATR-72). Using the FAST-OAD framework developed by ISAE-SUPAERO and ONERA, the objective is to perform high-fidelity MDAO to identify optimal configurations maximizing CO2 emissions reduction potential across a reference design mission profile.